HIV-1 primary isolates and cell line adapted HIV-1 strains differ in several aspects such as sensitivity to neutralizing antibodies and cellular tropisms. Earlier studies of HIV-1 neutralization were largely based on cell line adapted viruses. It has become apparent that the experience gained from the study of cell line adapted virus may not extend to the neutralization characteristics of primary isolates. Antisera generated by immunogens from cell line adapted viruses in general are ineffective against HIV-1 primary isolates. Unlike cell line adapted viruses, the neutralization determinants of primary isolates are largely unknown. In the case of the primary isolate DH012, infected chimpanzees mount potent neutralizing antibodies against this virus. The availability of potent neutralizing sera and a molecular clone of this virus offer an exceptional opportunity to study the neutralization determinants of this primary isolate. Our preliminary studies suggest that the neutralization determinants of DH012 are different from those of cell line adapted viruses such as HIV-l LAI, i.e. linear determinants, such as V3, do not appear to be responsible for the neutralizing activity in the anti-DH012 sera. The major goal of this study is to identify the neutralization determinants of DH012. Two approaches will be used to map the principal neutralization determinants (PND) in DH0l2. l. Chimeric viruses with DH012 envelope subfragments in the genetic background of NL4-3 virus will be made to identify the region(s) responsible for sensitivity to the DH012 neutralizing sera. NL4-3 is resistant to the DH012 neutralizing sera. 2. Development of resistant mutants using the anti-DH0l2 chimpanzee sera. The two approaches should complement each other in mapping the neutralization determinants, especially if the epitope contains discontinuous regions of the envelope glycoprotein. Once the neutralization determinants have been identified, we will study the antigenic properties of immunogens containing the DH012 PND with the emphasis on testing their ability to induce neutralizing immune responses to the virus. This study will provide the opportunities to explore the neutralization characteristics of HIV-1 primary isolates and contribute insights into strategies for the induction of polyclonal antibody responses which will neutralize primary isolates.